In the manufacture of nuclear fuel tubes, the fuel tubes are marked for identification with a bar code label which is laser marked onto the end of a fuel tube. The bar code label remains an identifying indicia of a particular fuel tube throughout the manufacturing process and in the loading of the reactor core. Typically, the fuel tubes are inserted into a laser marking chamber where they are laser marked in a predominantly oxygen atmosphere. The actual bar code label mark is produced during oxidation of the surface of the fuel tube when energy from the laser is directed onto the tube surface. This oxidized surface forming the bar code label typically is superficial. However, the actual penetration of the heat affected zone, is a critical parameter of the laser marking process and requires close monitoring. The heat affected zone is that depth into the tube wall surface which is affected by the laser energy and suffers some change in metal characteristics.
Typically, about no greater than twelve percent of the tube wall penetration of the heat affected zone is desirable. Any greater percentage of heat affected zone penetration is not desirable. Of secondary importance, is too shallow a depth of penetration of the heat affected zone. For example, if the oxygen content in the laser marking chamber is too low, such as when an oxygen gas line is broken or an oxygen gas flow valve is malfunctioning, the penetration of the heat affected zone will be shallow and the actual gray level of the bar code label will be too light in appearance, i.e. a low shade of gray, and impractical for positive identification during manufacturing and fuel tube loading. Sufficient heat was not generated in this instance. On the other hand, and of greater primary concern is the possibility of too deep a heat affected zone penetration. If the laser experiences excursions, such as resulting from power line fluctuations, laser lamp aging or laser tube to laser source geometry changes, the depth of the heat affected zone will be greater than desired, and as a result, the fuel tube structural integrity could be weakened. In such instances when the heat affected zone is greater than desired, the average gray level on the bar code label will be darker than the average gray level of a bar code label having an acceptable bar code label and depth of penetration of the heat affected zone.
Heretofore, current quality control and inspection techniques utilize destructive testing of laser marked tube samples and metallographic measurement of the heat affected zone. Typically, a tube sample from a random lot is broken and the tube sample having the bar code label thereon is etched and the depth of the heat affected zone measured under the microscope to monitor the laser marking. If a tube is determined as unacceptable, the laser or the oxygen content of the laser marking chamber is adjusted as necessary. This type of destructive testing is impractical because it results in the destruction of possibly good fuel tubes, and additionally, random sampling of fuel tubes for destructive testing is not a 100% tube verification system. There may be tubes that are not part of a sampled lot which suffer undesirable depth penetration of the heat affected zone. It is more desirable to test the tube with a known parameter such as the average gray level of the bar code label which changes to a darker level as the depth of the heat affected zone increases.
Therefore, it is an object of the present invention to provide a method and apparatus of monitoring the laser marking of a bar code label on a surface such as the end of a nuclear fuel tube for unacceptable depth penetration of the heat affected zone and which is non-destructive in nature and overcomes the deficiencies of the prior art.
It is still another object of the present invention to provide a method and apparatus for monitoring the laser marking of a bar code label on a surface such as the end of a nuclear fuel tube for unacceptable depth penetration of the heat affected zone and which utilizes the average gray level of the bar code label as a comparison with a known, predetermined standard.